Sodium- and potassium-dependent ATPase [(Na+ + K+)ATPase], which is responsible for the active transport of Na+ and K+, is distributed universally among animal cell membranes and consists of two types of subunits, alpha and beta (refs 1-4). The larger alpha-subunit with a relative molecular mass (Mr) of 84,000-120,000 is thought to have the catalytic role. We have now cloned and sequenced DNA complementary to the Torpedo californica electroplax messenger RNA encoding the alpha-subunit of (Na+ + K+)ATPase and have deduced the complete amino-acid sequence of the polypeptide. Some structural features of the alpha-subunit molecule related to the function of this active-transport protein are discussed.